Accumulators are devices that provide a reserve of hydraulic fluid under pressure. Accumulators are used in, for example, hydraulically-operated systems where hydraulic fluid under pressure operates a piece of equipment or a device. The hydraulic fluid may be pressurized by a pump that maintains the high pressure required.
If the piece of equipment or the device is located a considerable distance from the pump, for example, a significant pressure drop can occur in the hydraulic conduit or pipe which is conveying the fluid from the pump to operate the device. Therefore, the flow may be such that the pressure level at the device is below the pressure required to operate the device. Consequently, operation may be delayed until such a time as the pressure can build up with the fluid being pumped through the hydraulic line. This result occurs, for example, with devices located in a body of water at great depth, such as with a subsea test tree (“SSTT”) and blowout preventer (“BOP”) equipment, which is used to shut off a wellbore to secure an oil or gas well from accidental discharges to the environment. Thus, accumulators may be used to provide a reserve source of pressurized hydraulic fluid for such types of equipment.
In addition, if the pump is not operating, or if no pump is used, accumulators can be used to provide the source of pressurized hydraulic fluid to enable the operation of the piece of equipment or device.
Accumulators conventionally include a compressible fluid, e.g., gas such as nitrogen, helium, air, etc., on one side of a separating mechanism in a pressure resistant container, and a substantially incompressible fluid (e.g., hydraulic oil) on the other side of the separating mechanism. When the hydraulic fluid is released from the accumulator and the system pressure drops below the pressure on the gas side of the separating mechanism, the separating mechanism will move in the direction of the hydraulic fluid side of the separating mechanism, displacing the stored hydraulic fluid into the piece of equipment or the device as required.
When temperature changes within an accumulator, the precharge gas pressure will increase with increasing temperature and decrease with decreasing temperature. Changes in gas pressure affect the usable fluid volume that an accumulator can deliver. A near constant precharge pressure under varying temperatures would produce a near constant usable volume of fluid delivered by the accumulator. Accumulators known in the art use two chambers, one gas precharge chamber and on operating hydraulic fluid chamber. One solution to the problem of cooling of the gas pressure charge, and its consequent pressure reduction, is addressed in U.S. Patent Application Publication No. 2005/0022996A1, filed by Baugh and entitled, Temperature Compensation Of Deepwater Accumulators. The design disclosed in the Baugh publication includes heating of the gas by subsea heating elements to increase the temperature of the accumulator pre-charge gas.
There continues to be a need for improved temperature compensated accumulators.